1. Field of the Invention
This invention relates to masks used in photolithographic processes for manufacturing integrated circuits, and more particularly to methods for manufacturing masks used in x-ray photolithographic processes.
2. Description of the Prior Art
It is known in the art to use masks during the process of manufacturing integrated circuits. Typical steps taken during the manufacturing of integrated circuits involve coating a silicon wafer with a layer of photoresist, selectively exposing portions of the photoresist and removing the exposed portions. Thereafter, a number of other steps can be taken, e.g., doping the exposed portions of silicon with impurities or depositing metal over the exposed portions. Normally, the process of exposing photoresist involves the use of visible light. However, there is a limit to the resolution that can be achieved with light having a wavelength in the visible portion of the spectrum. It is known in the art that it is desirable to achieve very fine resolution when manufacturing integrated circuits because a finer resolution permits a circuit designer to design integrated circuits using less silicon surface area. This is desirable because processing silicon to make an integrated circuit is expensive and the smaller one can make an integrated circuit, the less expensive it will be.
One method of achieving fine resolution is to use x-ray radiation. An example of a system for manufacturing integrated circuits using x-ray photolithography is discussed in U.S. Pat. No. 3,743,842 (Smith et al.). Masks used in x-ray photolithographic processes typically have a coating which selectively blocks x-rays formed on an x-ray transparent membrane. Because the mask must be capable of producing repeatable geometries of submicron dimensions, the mask must be mechanically strong and constructed with very fine mechanical tolerances. The mask geometries must be capable of being aligned with another set of patterns on a wafer within submicron tolerances.